According to the conventional and widely used air-driven handpieces, pressurized air is impinged onto a wheel rotatably accommodated within a distal head of the handpiece to rotate the wheel and the cutting tool retained by the wheel. In those air-driven handpieces, the rotation of the wheel is maintained for a while by force of inertia even after the completion of the supply of the pressurized air. During the inertial rotation, although the air supply through the supply inlet is stopped, the air rotating with the wheel is forced out into the outlet, which in turn causes an interior of the head to be evacuated in part or in whole. This results in that contaminations adhering on or existing around the cutting tool such as saliva, blood, and/or cutting debris of the teeth may be sucked back through small gaps between the head and the tool into the interior of the head and then the air outlet, so that the contaminations would be accumulated within the interiors of the handpiece and air-supply tube, which has been known to the art as “sucback” or “sucking-back”.
For preventing the occurrence of such sucking-back, there have been proposed a various techniques so far. For example, Document 1 (JUM 6-20492 B) and Document 2 (JP 9-122146 A) disclose a technique for preventing the contamination of the handpiece, in which a low pressure air is fed into the de-energized handpiece to maintain a positive pressure within the interior of the handpiece. This, however, does not relate to the structural solution to the sucking-back problems.
Document 3 (JP 9-108239 A) discloses another technique in which a movable valve is mounted in the handpiece, by which the air discharge is halted with the stop operation of the wheel. The movable valve can be inoperative by the repetition of the sterilizations of the handpiece, which may fail to effectively attain the prevention of the sucking-back.
Document 4 (JP 2000-60870 A) discloses a handpiece in which a projection is provided on the upstream side of the air outlet with respect to the rotational direction of the wheel in order to prevent the pressurized air from being discharged into the air outlet. With the arrangement, the air outlet is opposed to the wheel, which unavoidably causes a negative pressure in the air outlet by the inertial rotation of the wheel and then is incapable of preventing the generation of the sucking-back effectively.
Document 5 (JP 6-28086 B) discloses a handpiece in which the contaminations such as cutting debris within the handpiece are discharged with an aid of centrifugal force through the contamination discharge pass into the atmosphere. Also, Document 6 (JP 2000-126203) discloses a handpiece in which a member is provided on the bearing for preventing the contaminations such as cutting debris from entering into the interior of the handpiece. These handpieces, however, are incapable of effectively preventing the invasions of the saliva and blood including contaminated small particles and mist thereof into the interior of the handpiece.